It is not there in the circuit diagram. It is better to add 4 diodes to 4 outputs to the keypad (RB0 ~ RB3) but things will work fine without that. It is just an additional protection.

Thank you very much for your valuable suggestion. Actually I didn’t even thought about this situation.

Even though I don’t want to complicate the circuit here as this is purely intended for hobbyists/students. I added a note below the circuit diagram adding these points. https://electrosome.com/matrix-keypad-pic-microcontroller/#Circuit_Diagram

I really appreciate your help.

You need to add diodes to the four outputs from the micro. With your current design, it is possible to put a short across the micro by simply pressing two buttons at the same time.

For example of you press both button 1 and button 2 at the same time you effectively join RB1 and RB2 together (follow the current path). When scanning RB1 you are driving it high while driving RB2 low, that results in a short circuit which will cause high current to flow through the port; this is likely to damage it permanently. Even if the micro has short circuit protection, it is bad practice to design any circuit that relies on such protection for normal operation. On many microcontrollers your circuit will simply destroy the output port if the user were to accidentally press two keys together.

or use a Texas Instruments TIL311 HEX LED display.

Yes, just turn on the led if it matches a particular key.

How may I use this matrix key to make on and off the led.

It is assembly language coding.